Attempts have been previously made to automate to various degrees the task of adding or removing sections to a drill string by means of a drilling apparatus such as a portable mobile drill unit. To complete the operation of adding sections it is typically necessary to have a storage rack to store a number of drill string sections, means to retrieve individual drill string sections from the storage rack, means to index the individual sections in line with the drill string axis, and means to couple a drill string section to the drill string. Removal of a drill string section from a drill string is accomplished by reversing the procedure utilizing the same apparatus.
It is still quite common for each drill string section to be manually placed for coupling in the drill string, thereby requiring the presence of someone on a continuous basis to add or remove the sections. Each section weighs approximately 60 pounds or more and may readily lead to operator fatigue.
In the typical application of a rock drilling unit, as an example, the drill string includes a number of drill string sections, typically each having a length of twelve feet. High pressure air is passed through the drill string sections to remove rock chips produced by the cutting face of the drill bit. Approximately 75% of the drill string sections employed in rock drilling have a hexagonal cross section, the typical drill string section having a one and one quarter inch hex. In the vast majority of applications, the rock drilling unit is employed to drill a hole of depth under forty feet. Therefore, the provision of storage rack holding four drill string sections, with one positioned on the drill string axis in the drill unit, will satisfy the vast majority of applications.
The typical service life of a drill string section varies in the range from 5000 to 20,000 feet of drilling. Unless provisions are taken to rotate the drill string sections, a drill string section adjacent the cutting head will reach the limit of its service life before drill string sections remote from the cutting head.
U.S. Pat. No. Re. 30,071 to Hilding et al discloses a drill string section handling apparatus. In that reference, the drill string sections are stored in an in-line manner in a magazine and transferred to the drill axis by means of a transfer arm. The drill section is held in the transfer arm by inclined rollers. The rollers are inclined at an angle which is comparable with the pitch angle of the thread on the drill string section and a number of the rollers are driven to rotate the drill section either into or out of the coupling sleeve on the section next ahead. In order to operate correctly, the drill string sections employed in the apparatus disclosed in this U.S. patent must have a round cross section to cooperate with the inclined rollers. As noted above, a majority of the drill string sections currently used have a hexagonal cross section. In addition, the in-line storage of the drill sections does not permit a change in the order of selection of the drill sections. Therefore, the first available drill section in the magazine is subject to more intensive use than those stored further back in the magazine.
A need has thus arisen for a drill string section changer that provides storage means, retrieving and indexing means with minimum operator input. In addition, a need has arisen for such an automatic changer adapted for use with a section having any crossectional configuration, and in particular in which hexagonal cross section drill string sections may be employed and in which the sections may be readily rotated in use to equalize the rate of wear.